1. Field of the Invention
The present invention relates to the field of circuits designed to mitigate the effects of exposure to high radiation levels. More specifically, the present invention relates to circuits designed to protect electrical systems during a dose rate event.
2. Description of the Related Art
The function of electrical circuits may be adversely affected by the effects of cosmic and nuclear radiation, which may manifest in the form of gamma rays, x-rays, and high energy particles such as neutrons, photons, electrons, ionized single particles, and beta particles. The rate at which these energetic particles are absorbed by an integrated circuit in a given environment is considered to be the dose rate, and is usually measured in rads in silicon per second (rad(Si)/sec). The dose rate of most environments is generally quite low and has little or no effect on the function of integrated circuits. However, aerospace environments are generally characterized with periods of elevated dose rates that may be several orders of magnitude larger than those experienced on the ground. Additionally, a nuclear weapon detonation creates an environment characterized by an extremely high dose rate delivered within a relatively narrow pulse width (on the order of nanoseconds). Integrated circuits designed to operate in both of these environments and other high dose rate environments must be able to withstand the adverse effects that accompany elevated absorbed dose rates.
Unlike single event effects, dose rate effects are not confined to a primary disruption of a single device in an integrated circuit. Additionally, unlike total ionizing dose effects, dose rate effects are generally transient events and are not the result of prolonged exposure to ionizing radiation environments. The intense bombardment of energetic and ionized particles during a dose rate event may generate electron-hole pairs within silicon and silicon-dioxide regions of integrated circuits. Excess charge created by this bombardment near semiconductor device junctions may be swept across the junction by electric field-induced drift and by carrier diffusion, resulting in a reverse leakage current in the junction. This leakage current, or photocurrent, may persist between a few nanoseconds to a few microseconds and comprise drift-induced current (prompt photocurrent) and diffusion current (delayed photocurrent). The total photocurrent produced at a junction is dependent on both the dose rate and junction characteristics, such as doping density. Generally, junctions with larger current ratings produce more photocurrent during a dose rate event.
Photocurrent generated in integrated circuit devices during a dose rate event may have several adverse effects on the functionality and integrity of the system. In the most extreme cases, the photocurrent itself may be sufficient to permanently or catastrophically damage integrated circuit components via a process termed burnout. During a burnout, the transient photocurrent may be sufficient to raise the temperature of metal layers in the integrated circuit to their melting point, thereby permanently damaging the circuit. Additionally, intense photocurrent may trigger a latchup in certain devices, which may result in a condition where an active device is fixed into a constant “on” position. In digital integrated circuits, the photocurrent can cause voltage glitches at nodes and may cause bit flips in memory elements, data latches, and shift registers. In linear circuits, photocurrents may cause output voltage transients that may last several milliseconds and adversely affect electrically coupled systems.
Previously, the problem of photocurrents in integrated circuits has been addressed by such circuit design techniques as current limiting. This method requires the use of additional active or passive components in order to prevent the creation of currents that are sufficiently large to cause burnout. In general, resistive elements are integrated into the system near device terminals in order to achieve this effect. However, adding resistive elements to the conductive paths flowing into device terminals may effectively increase the load experienced by each device, which may in turn decrease the overall speed of the system. Furthermore, during a dose rate event photocurrent flowing through current limiting devices may lower the voltage provided to system components, thereby lowering their upset threshold levels.
Other methods currently employed to counter the effects of photocurrents include using lower supply voltages, or increasing the time constant of circuits through the use of passive delay components. However, as a consequence both of these methods reduce the maximum speed of the system. Additionally, the use of lower supply voltages reduces the noise margins of devices in digital circuits making the system more susceptible to input signal voltage fluctuations.
The above methods for addressing the adverse affects of dose rates and photocurrents are generally limited in scope to decreasing the amplitude of photocurrents as experienced by individual system components. However, in sensitive digital systems or in linear circuits the presence of even slight voltage variations resulting from photocurrents may be sufficient to cause adverse behavior resulting in the processing or storage of corrupt data. As a result, simply reducing the strength of photocurrents generated by system components is not sufficient to adequately protect sensitive circuits during dose rate events. During a dose rate event, consideration must also be given to preventing current and voltage variations from being incorporated by digital processing and memory components, and manifesting as erroneously calculated or stored data.
In light of the above considerations, it would be desirable to have a dose rate protection circuit that can prevent a sensitive system from incorporating data corrupted by the effects of a dose rate event. Specifically, it would be desirable for this circuit to control a sensitive system such that the system selectively processes input signals depending on whether it is currently experiencing the effects of a dose rate event. Additionally, it would be beneficial for the circuit to be capable of providing a protective response commensurate with the strength of the dose rate event.